Method for capturing digital data in an automatic test system

Abstract

A method for operating automatic test equipment for capturing digital data produced by a semiconductor device under test, whereby the digital data is repetitively sampled to produce a series of sampled data pairs. The digital data and the sampling frequency can be non-coherent. As a result, the digital data can be sampled early relative to some bits and late relative to other bits. The sampled data pairs that are captured while these shifts take place, from early-to-late sampling or from late-to-early sampling, are then assigned to respective groups. A minimum number of bit patterns, corresponding to the sampled digital data, is then derived from contiguous groups of sampled data pairs, and compared with expected bit patterns. The method is especially useful for capturing digital data with drifting frequency.

What is claimed is: 
     
       1. A method of operating automatic test equipment to capture digital data produced by a semiconductor device under test, used for determining whether the device under test is functioning properly, comprising: (a) repetitively sampling the digital data to produce a series of sampled data pairs, each sampled data pair including a first data sample and a second data sample;   (b) assigning each sampled data pair in the series to one of a plurality of groups, each group including a plurality of sampled data pairs;   (c) designating the sampled data pairs in each group as occurring either early or late;   (d) deriving at least one serial bit pattern from contiguous groups of sampled data pairs, wherein the at least one serial bit pattern includes logical values of first data samples for each group of sampled data pairs designated as occurring late, and   wherein the at least one serial bit pattern includes logical values of second data samples for each group of sampled data pairs designated as occurring early; and     (e) comparing the at least one derived serial bit pattern with at least one expected serial bit pattern.   
     
     
       2. The method of operating automatic test equipment as recited in claim 1, wherein the assigning in step (b) includes the substeps of (b1) identifying a first plurality of sampled data pairs wherein the first data sample and the second data sample of each sampled data pair have different logical values, and   (b2) identifying a second plurality of sampled data pairs wherein the first data sample and the second data sample of each sampled data pair have the same logical value.     
     
     
       3. The method of operating automatic test equipment as recited in claim 2, wherein the assigning in step (b) further includes the substeps of (b3) identifying sampled data pairs in the first plurality that are preceded by runs having the same logical values as the first data samples in the identified sampled data pairs,   (b4) identifying sampled data pairs in the second plurality that are part of sufficiently long runs,   (b5) for each sampled data pair identified in step (b3), designating the last sampled data pair of an associated run as the first sampled data pair of a respective group,   (b6) designating each remaining sampled data pair in the first plurality as the first sampled data pair of a respective group, and   (b7) for each sampled data pair identified in step (b4), designating the first sampled data pair in the sufficiently long run as the first sampled data pair of a respective group.     
     
     
       4. The method of operating automatic test equipment as recited in claim 3, wherein the assigning in step (b) further includes the substeps of (b8) identifying designated first sampled data pairs that are followed by sampled data pair sequences having logical values (LL)(HL), (HH)(LH), (LH)(LL), (HL)(HH), (HH)(LL), or (LL)(HH),   (b9) identifying designated first sampled data pairs that are followed by sufficiently long runs,   (b10)for each designated first sampled data pair identified in step (b8), designating the first sampled data pair in an associated sequence as the last sampled data pair in the respective group, and   (b11)for each designated first sampled data pair identified in step (b9), designating the sampled data pair immediately preceding the sufficiently long run as the last sampled data pair in the respective group.     
     
     
       5. The method of operating automatic test equipment as recited in claim 1, wherein the designating in step (c) includes the substeps of (c1) identifying groups that begin with sampled data pair sequences having logical values (LL)(HL) or (HH)(LH),   (c2) designating each group identified in step (c1) as starting late, and   (c3) designating each remaining group as starting late or early.     
     
     
       6. The method of operating automatic test equipment as recited in claim 5, wherein the designating in step (c) further includes the substeps of (c4) identifying groups that end with sampled data pair sequences having logical values (LH)(LL) or (HL)(HH),   (c5) designating each group identified in step (c4) as ending early, and   (c6) designating each remaining group as ending early or late.     
     
     
       7. The method of operating automatic test equipment as recited in claim 6, wherein the designating in step (c) further includes the substeps of (c7) counting the number of sampled data pairs in each group,   (c8) counting the number of sampled data pairs between adjacent groups, and   (c9) determining whether the designations made in steps (c2), (c3), (c5), and (c6) are valid based on the number of sampled data pairs in each group and the number of sampled data pairs between adjacent groups.     
     
     
       8. The method of operating automatic test equipment as recited in claim 1, wherein the deriving in step (d) includes the substeps of (d1) identifying groups that start early and end early,   (d2) identifying groups that start late and end late,   (d3) identifying groups that start late and end early,   (d4) identifying groups that start early and end late,   (d5) designating the sampled data pairs in each group identified in step (d1) as occurring early,   (d6) designating the sampled data pairs in each group identified in step (d2) as occurring late,   (d7) designating the last sampled data pair in each group identified in step (d3) as occurring late and early and designating the remaining sampled data pairs as occurring late, and   (d8) designating the sampled data pairs in each group identified in step (d4) as occurring early and ignoring the last sampled data pair.     
     
     
       9. A method of operating automatic test equipment to capture drifting digital data produced by a device under test, comprising: (a) repetitively sampling the digital data to produce a series of sampled data pairs, each sampled data pair including a first data sample and a second data sample;   (b) assigning each sampled data pair in the series to one of a plurality of groups, each group including a plurality of sampled data pairs;   (c) designating corresponding first and last sampled data pairs in each group as having been acquired either early or late relative to an associated bit in the digital data;   (d) determining whether the designations of step (c) are valid relative to a measured amount of drift, and eliminating invalid designations;   (e) designating corresponding sampled data pairs in each group as having been acquired either early or late relative to an associated bit in the digital data based on the valid designations of step (c);   (f) deriving at least one serial bit pattern from contiguous groups of sampled data pairs; and   (g) comparing the at least one derived serial bit pattern with at least one expected serial bit pattern.   
     
     
       10. The method of operating automatic test equipment to capture drifting digital data as recited in claim 9, wherein the assigning of step (b) includes the substeps of (b1) identifying sampled data pairs that occur during bit transitions,   (b2) for the sampled data pairs identified in step (b1) that are preceded by runs having the same logical values as the first data samples of the identified sampled data pairs, designating the last sampled data pairs in the runs as the first sampled data pairs of respective groups, and   (b3) for the sampled data pairs identified in step (b1) that are not preceded by runs having the same logical values as the first data samples of the identified sampled data pairs, designating the identified sampled data pairs as the first sampled data pairs of respective groups.     
     
     
       11. The method of operating automatic test equipment to capture drifting digital data as recited in claim 10, wherein the assigning of step (b) further includes the substeps of (b4) identifying sampled data pairs that do not occur during bit transitions, and   (b5) for the sampled data pairs identified in step (b4) that are part of sufficiently long runs, designating the first sampled data pairs in the runs as the first sampled data pairs of respective groups.     
     
     
       12. The method of operating automatic test equipment to capture drifting digital data as recited in claim 11, wherein the assigning of step (b) further includes the substeps of (b6) for the sampled data pairs identified in steps (b1) and (b4) that are followed by sampled data pair sequences having logical values (LL)(HL), (HH)(LH), (LH)(LL), (HL)(HH), (HH)(LL), or (LL)(HH), designating the first sampled data pairs in the sequences as the last sampled data pairs of the respective groups, and   (b7) for the sampled data pairs identified in steps (b1) and (b4) that are followed by sufficiently long runs, designating the sampled data pairs immediately before the runs as the last sampled data pairs of the respective groups.     
     
     
       13. The method of operating automatic test equipment to capture drifting digital data as recited in claim 12, wherein the assigning of step (b) further includes the substep of (b8) assigning the sampled data pairs between each designated first sampled data pair and last sampled data pair to the same respective groups.     
     
     
       14. The method of operating automatic test equipment to capture drifting digital data as recited in claim 9, wherein the designating of step (c) includes the substeps of (c1) identifying groups that start with sampled data pair sequences having logical values (LL)(HL) or (HH)(LH),   (c2) identifying groups that end with sampled data pair sequences having logical values (LH)(LL) or (HL) (HH),   (c3) designating the groups identified in step (c1) as starting late,   (c4) designating remaining groups as starting early or late,   (c5) designating the groups identified in step (c2) as ending early, and   (c6) designating remaining groups as ending early or late.     
     
     
       15. The method of operating automatic test equipment to capture drifting digital data as recited in claim 9, wherein the measured amount of drift is proportional to either the minimum number of sampled data pairs between a first sampled data pair designated as having been acquired early and a second sampled data pair designated as having been acquired late, or the minimum number of sampled data pairs between a third sampled data pair designated as having been acquired late and a fourth sampled data pair designated as having been acquired early.   
     
     
       16. The method of operating automatic test equipment to capture drifting digital data as recited in claim 15, wherein the determining of step (d) includes the substeps of (d1) counting the number of sampled data pairs in each group,   (d2) counting the number of sampled data pairs between adjacent groups, and   (d3) determining whether the designations made in steps (c3), (c4), (c5), and (c6) are valid based on the number of sampled data pairs in each group, the number of sampled data pairs between adjacent groups, and the measured amount of drift.     
     
     
       17. The method of operating automatic test equipment to capture drifting digital data as recited in claim 9, wherein the designating of step (e) includes the substeps of (e1) for each group designated as starting early and ending early, designating each sampled data pair in the group as occurring early,   (e2) for each group designated as starting late and ending late, designating each sampled data pair in the group as occurring late,   (e3) for each group designated as starting late and ending early, designating the last sampled data pair in the group as occurring late and early and the remaining sampled data pairs as occurring late, and   (e4) for each group designated as starting early and ending late, ignoring the last sampled data pair in the group and designating the remaining sampled data pairs as occurring early.     
     
     
       18. The method of operating automatic test equipment to capture drifting digital data as recited in claim 9, wherein the deriving in step (f) includes the substeps of (f1) storing the first data sample and ignoring the second data sample of each sampled data pair designated as occurring late,   (f2) storing the second data sample and ignoring the first data sample of each sampled data pair designated as occurring early, and   (f3) storing the first data sample and the second data sample of each sampled data pair designated as occurring late and early.